Vapor traps

ABSTRACT

In a refrigerated vapor trap for a vapor vacuum pump, the refrigerated trapping surfaces are arranged to be screened from external heat sources by additional water-cooled surfaces so that should the supply fail of the refrigerating liquid (such as liquid nitrogen) then the trapping surfaces reach a temperature which is still below ambient.

o mted States Patent [15] 3,o35,o39 Power et al. Jan. 18, 1972 [54]VAPOR TRAPS 3,296,810 1/1967 l-lablanian ..62/55.5 [72] Inventors: BullD. Power, Horsham; Roger D. 2'934257 4/1960 Power 7 3,019,809 2/1962lpsen f gng'and 3,081,068 3/1963 lnil1eron.. [73] Assignee: The BritishOxygen Company Limited, 3,137,551 6/ 6 Mark C 'a Iey England 1 Boyer[22] Filed: 1970 Primary Examiner-William J. Wye 21 Appl 32,452AttorneyTownshend&Meserole [57] ABSTRACT [30] Foreign ApplicationPriority Data In a refrigerated vapor trap for a vapor vacuum pump, theApr. 28, 1969 Great Britain ..21602 f i t d trapping Surfaces arearranged to be screened from external heat sources by additionalwater-cooled sur- [52] US. Cl. ..62/55.5 faces so that should the supplyfaii of the refrigerating liquid [51] Cl Bold 5/00 (such as liquidnitrogen) then the trapping surfaces reach a [5 8] Fleld oi Search..62/55.5 temperature which is Stiii below ambient [56] References Cited11 Clairns Eigures UNITED STATES PATENTS 3 ,321,927 5/1967 Hood 6 2 5. 5

PATENTEUJAIIBIIR 3.635.039

SHEET 1 OF 2 FlG.l

INVENTOR 805/5 D- ATTORNEYS PAIENIEBJmmm slsas'oss SHEET 2 BF 2 4 5 g gi l I A FIG.2

INVENTOR B19: 1' D.Po s1? AT TORNEYS VAPOR This invention relates tovapor traps, particularly for high vacuum systems employing vapor vacuumpumps.

In order to stop vapor from the vapor vacuum pump passing to theequipment being evacuated, it is known to interpose a refrigerated trapbetween the pump and the equipment. The refrigerant for high-vacuumsystems is usually liquid nitrogen, which is arranged to abstract heatfrom an array of surfaces in the vapor path between the pump and theequipment. These trapping surfaces reach such a low temperature that anyincident vapor particles freeze on to them.

The trap is usually supplied intennittently with liquid nitrogen, as bebeing topped-up manually. This has the disadvantage that whenreplenishing of the refrigerant (or, more accurately, cryogenic) liquidis discontinued, the trap tends to warm up towards the local ambienttemperature, thus both releasing frozen vapor and failing to trap freshvapor emitted by the pump.

It is known to attempt to overcome this problem by placing water-cooledbaffles in the vapor path, so that the vapor condenses on them whenthere is no cryogenic liquid in the trap. This has the disadvantage thatthe trapping surfaces when combined with the baffles impede to anundesirable extent the flow of the fluid being evacuated.

The present invention aims at providing a refrigerated vapor trap inwhich the trapping surfaces are continuously maintained at a temperaturegenerally below ambient, even in the absence from the trap of a crogenicliquid.

Accordingly the present invention provides a refrigerated vapor trap fora vapor vacuum pump, which is as claimed in the appended claims.

The present invention will now be described by way of example withreference' to the accompanying drawing in which:

FIG. 1 is a diagrammatic sectional view of one form of trap according tothe invention;

FIG. 2 is a diagrammatic view of a modified housing, and

FIG. 3 is a diagrammatic view of a further modified housing.

In a known form of refrigerated trap a set of trapping surfaces are innormal contact with a support member having in its two coolant passages.One passage is for liquid nitrogen or other cryogenic (or refrigerant)liquid, and the other is for tap water, or other fluid coolant, of whichthe temperature is normally below ambient. Before this known trap isoperated, the water in the respective coolant passage has to be frozenso that there is no flow of water through the passage, while thecryogenic liquid flows through the other passage to keep the varioustrapping surfaces associated with it at a very low temperature. Thisstate can be reached only by stopping the flow of water initially andusing the liquid nitrogen to freeze the static water. Thereafter a valveor tap in the water passage has to be opened, but there is of course noensuing flow because of the ice blockage. Thus when the apparatus isworking normally, the trapping surfaces act to capture by freezing anyvapor particles incident upon them.

When the flow or supply of liquid nitrogen is discontinued and theremaining nitrogen vaporized by head inleak, the temperature of thetrapping surfaces starts to rise. This can continue until it reachessuch a value that the frozen water in the support member is thawed. Thetrapping surfaces thereafter remain at about the temperature of thecooling water, which is now free to flow through the support member.Under these conditions the trapping surfaces are acting as water-cooledsurfaces on which the vapor can condense and flow back into the vaporvacuum pump.

When fresh nitrogen or other cryogenic liquid is supplied, it isnecessary for the flow of water to the support member to be stopped fora time, as the nitrogen cannot abstract enough heat to freeze the waterwhen flowing.

This known arrangement has the disadvantages that the operator has toremember both to close and to open the water valve at the appropriatetimes; that the cyclic freezing and thawing of the support member, withthe concomitant contraction and expansion thereof, require a trap whichis more robust and massive than is necessary functionally, and that thetrap has to be in a cooling water circuit different from that by whichthe vapor vacuum pump is cooled.

In the trap of the present invention, the flow of cooling water iscontinuous, and can be in series with the flow of cooling water to thevacuum pump. There is thus no need for the operator to concern himselfwith opening and closing the water valve at daily or other intervals,and because the cooling water is never frozen the trap can be oflightweight construction.

The refrigerated trap shown in the accompanying drawing includes anannular reservoir 2 for liquid nitrogen or other cryogenic liquid. Thereservoir has a central cylindrical passage d in which is positioned anarray of members 6 providing the trapping surfaces. These members 6 aresupported at the lower end of the passage 4 by supports 8 of which onlytwo are shown. This support is in good mechanical and thermal contactwith the reservoir and members 6, and serve as a heat transfer pathbetween the trapping surfaces and the reservoir 2. The reservoir,trapping surfaces, and supports are of stainless steel of which theouter surfaces are preferably treated, as by being polished, so thatthey are of low absorptivity so as to reduce heat transference byradiation from the surrounding housing and adjacent heat sources to thereservoir and the cryogenic liquid. In order to reduce undesired heatinleak to the reservoir 2 by conduction the reservoir inlet pipe 12(which is also made of stainless steel) is relatively long and isthin-walled. If it is not sufficiently strong to support by itself thereservoir when full with liquid nitrogen against the stresses imposed onit in use, then reinforcing ties or struts of low thermal conductivitymay extend between the reservoir and the adjacent housing, but these areomitted from the drawing for clarity. Encasing the reservoir 2 is ahousing 114 of hollow cross section so that it is close to, but spacedfrom, the cylindrical outer surface and axially directed end surfaces ofthe reservoir. The housing 14 has wrapped around it a tubular coil 16,of which the inlet 18 is intended to be connected to a tap, and of whichthe outlet 20 is intended to be connected to the cooling coil of anassociated vapor vacuum pump (not shown) In use, the refrigerated trap(including essentially the reservoir 2 and the housing M) is sealed tothe upper end of the pump. The equipment (also not shown) to beevacuated is in fluidtight engagement with the upper surface of thehousing 14.. With this arrangement the fluid to be evacuated from theequipment passes downwardly through an opening 22 in the housing 14(which opening is aligned with the passage 4 in the reservoir 2), passesover the trapping surfaces 6, over a set of inclined louvres 24 inthermal contact with the housing M, and through a lower opening 26 inthe housing.

The axes of the louvres 2d are parallel to each other, and the louvresextend across the mouth of the opening 26 and are inclined andpositioned so that they prevent the trapping surfaces 6 from receivingheat by radiation except along specified directions. These louvres 24are in thermal contact with the housing 14, so that they are maintainedsubstantially at the temperature of the cooling water. They thus act aswatercooled surfaces on which vapor from the vacuum pump can condense,but their primary function is to act as radiation shields. Thus theyimpede the flow of fluid to the pump from the equipment to a lesserextent than they would if designed as true baffles. With thisarrangement, the trapping surfaces 6 can see only the cooled surfaces ofthe associated vacuum pump, and are not able to receive heat byradiation directly from the hot jet assembly of the vacuum pump.

In spite of the good thermal insulation of the reservoir and trappingsurfaces from the housing, the almost complete encasement of thereservoir and surfaces by the outer housing and screen (kept at thetemperature of the cooling water), plus the fact that allinterconnecting ducts and supports are kept at cooling watertemperature, ensure that, in the absence of cryogenic liquid, thetemperature of the reservoir and trapping surfaces tends towards that ofthe cooling water and not to that of any adjacent heat sources, whichmight be at significantly higher temperature.

When the apparatus is working, the space between the reservoir 2 and thehousing 14 is automatically evacuated by the vapor vacuum pump so thatthe reservoir is able to take up heat from the housing 14 substantiallyonly by radiation through the space, and by conduction along the inletpipe 12. The radiation heat inleak is kept to a low figure by causingthe respective surfaces of the housing to have low emissivity: this canbe done by polishing the surfaces which are preferably of stainlesssteel.

When the apparatus is working, the continuous flow of tap water throughthe coil 16 keeps the housing 14 at a temperature which is generallybelow the local ambient temperature.

Owing to the very small heat transfer from the housing to the reservoir,there is no danger of the water flowing through the coil 16 beingfrozen. The liquid nitrogen in the reservoir 2 slowly absorbs heat andis vaporized, but as long as there is any liquid nitrogen in thereservoir then the reservoir is substantially at the temperature ofliquid nitrogen, i.e., l96 C. When stable operating conditions arereached, the trapping surfaces 6 assume this temperature (or come veryclose to it), whereas the louvres 26 are substantially at thetemperature of the cooling water.

in operation of the vacuum vapor pump, vapor from the operating fluid,e.g., oil or mercury, tends to back-stream towards the equipment beingevacuated. In doing so it passes through the louvres 26, which aresufficiently cold for a proportion of the vapor to condense on them andto drop back into the pump. However the remaining portion of the vaportends to impinge on the trapping surfaces 6 to which the vapor particlesimmediately freeze, so that the space above and trapping surfaces, andparticularly including the evacuated equipment, is virtually free ofcontamination by the vapor.

When all the liquid nitrogen in the reservoir 2 has vaporized (which canhappen overnight or over a weekend when the equipment is not being used)the inleak of heat to the reservoir gradually raises its temperature.The present invention aims at keeping this undesired heat inleak to avery low value. By far the major portion of the surface area of thereservoir and its associated trapping surfaces is able to receive heatonly from the housing 14. The remaining surfaces of the reservoir, whichin practice means substantially only the inner cylindrical surfaceforming the passage 4, and the trapping surfaces 6, can receive acertain amount of heat through the openings 22 and 26, but the solidangles through which they can receive heat from such external heatsources are relatively small. This has the effect that the temperatureof the trapping surface 6 in particular can rise only as high as (orvery little higher than the temperature of the cooling water, andtherefore stays below ambient temperature. If this warming-up of thereservoir and trapping surfaces is allowed to continue, the temperatureeventually stabilizes at about that of the cooling water, so that thetrapping surfaces 6 come to function more as water-cooled surfaces thanas refrigerated surfaces. This resultant decreased efficiency oftrapping of the vapor particles is acceptable over the period when theapparatus is not used, and is still sufiicient to prevent grosscontamination of the interior of the equipment by condensed vapor.

For some applications of the present invention it might be sufficientfor the user not to suppiy cryogenic liquid, but to rely on the trappingsurfaces 6 and the louvres 24 to keep the amount of vapor reaching theinterior of the equipment at an acceptable value. However for greaterefficiency of trapping, it would be necessary to use liquid nitrogen orother cryogenic liquid.

As shown in FIGS. 2 and 3 of the drawing, the solid angle through whichheat can be radiated to the reservoir and trapping surfaces can bereduced still further, such as by extending the housing 14appropriately. In FIG. 3 the housing has a portion 28 extendingvertically upwardly to define an elongated opening 22, As shown in FIG.2, the housing can be provided with a reentrant skirt 30 effectivelyscreening the inner cylindrical surface of the reservoir. Both orsimilar modifications can be made if desired.

In FIGS. 2 and 3 the trapping surfaces and radiation shields have beenremoved for clarity.

Other modifications lying within the scope of the present invention canbe used. Thus although the water-cooled louvres 24 have been stated ashaving their axes parallel with each other, they could take the form ofa series of concentric frustoconical surfaces. Conversely, although thetrapping surfaces 6 are shown as being in the form of such frustoconicalmembers, they could take the form of parallel inclined slats arranged sothat there is no linear path through the passage 4 which does notintersect the trapping surfaces, reservoir, or extensions thereof.

It will thus be seen that the trap of the present invention requires nosequential operation of water taps when it is to be used with cryogenicliquid, and that the trap can be of a relatively lightweightconstruction dictated more by its function than by the necessity to bemechanically robust.

We claim:

1. A refrigerated vapor trap for a vapor vacuum pump, including areservoir or duct for liquid refrigerant, an internal passage in whichis an array of trapping surfaces in thermal contact with the reservoiror duct, a housing encircling and spaced from the reservoir or duct andadapted to be cooled with the fluid coolant to a temperature nearer butbelow ambient, and additional fluid-cooled surfaces, not encircling thereservoir or duct, positioned on the same side of the trapping surfacesas that on which the vacuum pump is or is to be, positioned whereby, inthe absence of the refrigerant liquid, the fluid-cooled housing andadditional surfaces screen the trapping surfaces from external heatsources so that the trapping surfaces tend to assume a temperature whichis nearer to that of the fluid coolant than to that of the ambientatmosphere.

2. The vapor trap as claimed in claim 1', in which the said additionalsurfaces consist of, or include, a louvered screen which extends acrossat least that end of the said passage which is nearer the vacuum pumpwith which the trap is to be used, so that the screen acts as aradiation shield, the screen being in thermal contact with the fluidcoolant.

3. The vapor trap as claimed in claim 2, in which the louvres of thescreen have their longitudinal axes extending in parallel with eachother.

4. The vapor trap as claimed in claim 1, in which at least some of thetrapping surfaces are provided by an array of frustoconical membersspaced apart along the axis of the said passage.

5. The vapor trap as claimed in claim 1, in which the housing directlysupports the said reservoir.

6. The vapor trap as claimed in claim 1, in which the said reservoir isin the fonn of an annular container having a right cylindrical centralpassage.

7. The vapor trap as claimed in claim 6, in which the reservoir,trapping surfaces and supports therefore extending between the reservoirand the trapping surfaces, are of stainless steel.

8. The vapor trap as claimed in claim 7, in which the external stainlesssteel surfaces are polished.

9. The vapor trap as claimed in claim 1, in which the additionalsurfaces are provided by extensions from the housing which reduce thesolid angle through which radiated heat can be received by the reservoiror trapping surfaces from externally of the trap.

10. The vapor trap as claimed in claim 9, in which one additionalsurface is provided by a reentrant skirt which is spaced inwardly fromthe surfaces of the passage in the reservoir.

11. The vapor trap as claimed in claim 9, in which one or anotheradditional surface is provided by a cylindrical exten-' sion thereofcoaxial with the passage in the reservoir and forming an extensionthereof.

1. A refrigerated vapor trap for a vapor vacuum pump, including areservoir or duct for liquid refrigerant, an internal passage in whichis an array of trapping surfaces in thermal contact with the reservoiror duct, a housing encircling and spaced from the reservoir or duct andadapted to be cooled with the fluid coolant to a temperature nearer butbelow ambient, and additional fluidcooled surfaces, not encircling thereservoir or duct, positioned on the same side of the trapping surfacesas that on which the vacuum pump is or is to be, positioned whereby, inthe absence of the refrigerant liquid, the fluid-cooled housing andadditional surfaces screen the trapping surfaces from external heatsources so that the trapping surfaces tend to assume a temperature whichis nearer to that of the fluid coolant than to that of the ambientatmosphere.
 2. The vapor trap as claimed in claim 1, in which the saidadditional surfaces consist of, or include, a louvered screen whichextends across at least that end of the said passage which is nearer thevacuum pump with which the trap is to be used, so that the screen actsas a radiation shield, the screen being in thermal contact with thefluid coolant.
 3. The vapor trap as claimed in claim 2, in which thelouvres of the screen have their longitudinal axes extending in parallelwith each other.
 4. The vapor trap as claimed in claim 1, in which atleast some of the trapping surfaces are provided by an array offrustoconical members spaced apart along the axis of the said passage.5. The vapor trap as claimed in claim 1, in which the housing directlysupports the said reservoir.
 6. The vapor trap as claimed in claim 1, inwhich the said reservoir is in the form of an annular container having aright cylindrical central passage.
 7. The vapor trap as claimed in claim6, in which the reservoir, trapping surfaces and supports thereforeextending between the reservoir and the trapping surfaces, are ofstainless steel.
 8. The vapor trap as claimed in claim 7, in which theexternal stainless steel surfaces are polished.
 9. The vapor trap asclaimed in claim 1, in which the additional surfaces are provided byextensions from the housing which reduce the solid angle through whichradiated heat can be received by the reservoir or trapping surfaces fromexternally of the trap.
 10. The vapor trap as claimed in claim 9, inwhich one additional surface is provided by a reentrant skirt which isspaced inwardly from the surfaces of the passage in the reservoir. 11.The vapor trap as claimed in claim 9, in which one or another additionalsurface is provided by a cylindrical extension thereof coaxial with thepassage in the reservoir and forming an extension thereof.